Migrate to the schema-3 dependency set with documents.js 2.0.0

[Mearman]

documents.js 2.0.0 changed the DocumentPackage shape (the stored layout half is gone; per-node frames and package pages carry the geometry) and moved the whole family onto document-schema.js 3.x, where heading paragraphs carry headingLevel in the schema itself. This app holds documents.js, markdown-codec, and odf.js in one tree, so the single-package bump automation could never go green on it (sibling-update/documents.js-2.0.0 is red on Typecheck for exactly this reason) -- this is the one coordinated migration PR.

What changed:

• Dependencies: documents.js ^1.102.2 -> ^2.0.0 (markdown-codec ^2.0.0 and odf.js ^3.0.1 were already in). The lockfile had document-schema.js 3.1.0 (via markdown-codec/odf.js) alongside 3.2.0 (via documents.js) after the bump; deduped so exactly one schema version (3.2.0) resolves across the whole tree.
• Test fixtures: contentCounts.test.ts hardcoded formatVersion: 2; it now asserts the exported CONTENT_FORMAT_VERSION so it tracks the schema major instead of breaking on the next one.
• Heading detection (src/rpc/router.ts): the app's heading-{N} styleId convention is now minted from ContentParagraph.headingLevel -- markdown-codec 2.0.0's lowering, ooxml.js's docx reader (w:outlineLvl through the style chain), and odf.js's odt reader (text:outline-level) all populate it. The docx/odt path keeps the Heading{N} styleId pattern as a fallback because the signals have different coverage: a style named Heading3 with no outline level is caught only by the pattern, and a custom style inheriting an outline level with a non-Heading name is caught only by the field. The markdown path drops parseHeadingStyleId entirely -- headingLevel is authoritative there.
• xlsx content reads: documents.js 2.0.0 re-exports readXlsxContent, so the xlsx branch of readContentForFormat no longer detours through a full xlsx->ods bridge conversion to read .content off the result -- it uses the direct package reader like every other format, which also makes the function synchronous and drops its unused signal parameter.

The app never constructs a DocumentPackage or reads a stored .layout half itself (the pdf-inspect tool's LayoutDocument comes from readPdf, pdf-codec's own contract, which is unchanged), so the package-shape change needed no further code moves beyond the above.

Supersedes #199 (the automation's documents.js 2.0.0 bump PR, red on Typecheck).

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